A combination of unpredictable oil prices, global warming and an ageing, often ill-planned, electricity grid system in countries such as the USA and UK places many demands on electricity generation and distribution. A particular problem is that of how to ensure supply meets demand. This is because electricity networks often have little or no electricity storage capacity. Hence, the demand at any time must be forecast and generation resources brought on and off line accordingly. This results in the need for extra generation capacity in the system, so called spinning reserve.
There are a few options currently available for large-scale storage of electricity, for example pumped hydro schemes. These generally involve construction of a dam and reservoir at altitude with a hydro generating facility at a lower altitude. During times of low demand, electricity is taken from the grid and used to pump water up into the reservoir, which is then released to turn turbines during time of peak demand. However, these systems require certain geological features that limit the scope for their application. In addition, there can be adverse environmental impact such as flooding of land.
Another storage solution uses battery arrays. These are generally suited as small scale localised back up, such as for uninterrupted power supplies (UPS) for key installations or delicate equipment such as hospitals or computer servers. Large arrays of nickel metal hydride (NiMH) batteries have been used for grid backup for isolated communities, such as that recently trialled in Fairbanks, Ak. However, these are expensive to install, require the maintenance and monitoring of many cells, and generate significant amounts of low-grade heat that is difficult to utilise. They can also suffer from self-discharge and have a limited cycle life.
A further option is to use flow cells, often called redox batteries. These use the constant circulation of two electrolytes, each containing one half of a reversible redox couple through electrical cells. Electricity may be stored by driving a current through the cells in order to electrochemically reduce one electrolyte whilst oxidising the other. When current is required, the applied current is removed and the reduction/oxidation process reverses to provide a current. Whilst these provide an efficient and flexible large-scale power storage solution, they have some significant disadvantages. For example, some use hazardous materials requiring extensive and expensive safety systems. Also, some suffer from a low energy density due to poor solubility of the reactive components, mass transport effects due to the active materials being in solution and problems due to self-discharge caused by active material crossing the cell membranes.
Another storage solution uses fuel cells, which electrochemically react two fuel materials that are continuously supplied to the cell in order to generate electricity. There are many possible electroactive couples that may be used in fuel cells such as ethanol/oxygen and methane/oxygen. The most environmentally attractive is the hydrogen/oxygen cell. This utilises the electrochemical reaction of hydrogen and oxygen to produce water with heat as a by-product. Advantageously, all the products produced are usable and generally environmentally benign.
Although some of the reactants used in fuel cells, such as oxygen, may be extracted from the atmosphere, others, such as hydrogen, need to be produced. This may be done on site or off site with the hydrogen being transported to its location of use, often in liquefied form. One common method of hydrogen production is electrolysis of water, which generally involves passing a current between catalytically active electrodes in order to split it into hydrogen and oxygen. The hydrogen is dried and compressed into storage tanks. When needed, the hydrogen is passed to the fuel cell where it is electrochemically reacted with oxygen from air to give electricity. This is, however, a reasonably complex arrangement, as it needs a pure water source, an electrolyser, compressors, driers, storage tanks, fuel cell and its ancillaries, and enough sensors and control gear to integrate the system.